Continuous cheese-making process utilizing an immobilized rennet enzyme reactor

ABSTRACT

Cheese is made continuously by flowing milk through a spiral flow path of a reactor containing an immobilized enzyme such as rennet which coagulates the milk. The spiral flow path is formed by a spirally wound microporous sheet containing the immobilized enzyme. Adjacent surfaces of the sheet are spaced apart with a spacer which can be a plurality of microporous ribs or a sheet in open net form.

This application is a continuation of application Ser. No. 06/641,445,filed Aug. 16, 1984, now abandoned.

This invention relates to an improved method for forming cheese in acontinuous manner. More particularly, this invention relates to acontinuous process for making cheese starting at low and up to hightemperatures, e.g. up to 40° C., or a combination of the two in steps tocause clotting and cheese curd formation. Still further, this inventionrelates to a method whereby repeat treatment of partially or fullyreacted milk is utilized in a cheese-making process includingreutilization of any residual unreacted values which may further beexhausted of any valuable constituents thereof.

BACKGROUND OF THE INVENTION

In a typical cheese-making operation, milk is inoculated with anappropriate culture often called a starting culture or bacterialstarter. The bacterial starter causes acidity to develop in the cheeseas a result of lactic acid formation. The various bacterial additionsalso produce various cheese types. Thereafter the thus inoculated milkis contacted by stirring with an appropriate amount of enzyme rennetused to coagulate the casein in the milk to produce the cheese curds.While coagulation takes place, the vessel contents are not stirred. Thisbatch reaction may be conducted in another tank although the same tankmay be utilized. Thereafter stirring is resumed; curd formation takesplace, and whey is separated from the curds. Thereafter a salt wash maybe used to separate further the whey from the cheese. Whey, of course,is not affected by rennet.

Typically the feedstock is at a proper pH, e.g. about 6-6.5, andtemperature and thus produces the controlled quality of cheese sought tobe obtained. The present invention, however, does not relate to acidprecipitation such as at a pH of 4 to 5, e.g. for cottage cheese.

As it is evident from the above description, cheese-making isessentially a batch process wherein different vessels may also be usedto produce different reactions. These operations are described inliterature such as Van Slyke and Price, "Cheese", Orange Judd Pub. Co.,New York, NY (1949).

In an endeavor to reduce the large scale batch vessels or to speed upthe production of cheese, a number of batch or semi-batch operationshave been proposed.

One of the proposed methods requires an enzyme deposited on flatsurfaces. The substrate, that is the bacterially inoculated material, isflowed past the enzyme. Thus the substrate is being treated in acontinuous manner.

These surface deposited enzymes, of course, have imposed a greatlimitation on the flow rates, especially since the enzyme deposition hasbeen by adsorption on a surface when these enzymes have been sought tobe deposited on the surface.

Additionally, long pipe reactors have been proposed such as where rennetand milk are mixed in the pipe reactor, and then dumped in a vessel, ora long pipe reactor for a flow method in which the reaction would takeplace on the walls of the pipe containing the adsorbed enzymes.

According to the last method the bacterially inoculated material flowsthrough the pipes at low velocities so as not to strip, by turbulentflow, the enzymes deposited on the wall surfaces. As it is well known,at low velocities the laminar flow conditions obtain and the velocityprofile in a pipe reactor often resembles substantially a parabola. Asthe reaction rates are different based on the velocity profile as wellas the turbulence or lack thereof, the production quality or yield oftensuffered.

Additionally, various enzymes deposited on filter surfaces likewise havebeen sought to be immobilized by other enzyme carriers which wereretained on the filter material, e.g. due to mesh constraints or filtermaterial type for filter leaves of the apparatus. The fluid flow hadbeen sought to be such as to cause the reaction to be continuouslyconducted.

However, one of the great shortcomings of the prior art batch, wall, orleaf filter type of reactors has been the inexact rennet to milk ratiocaused by improper mixing, or clogging and/or stripping of thesesurface-deposited materials which all have affected the fairly sensitiverennet to milk ratios, thus adversely affecting the necessary quality,control and yields.

Moreover, only a substrate of a high degree of purity could be employedto produce the desired or sought-after enzymatic conversion atsacrifices in reaction rates and flow rates. As a general proposition,all substrate materials which carried particulates such as proteins,fats, coagulants, etc., would cause to blind, i.e. clog, the reactionsurfaces. Moreover, these particulates will also cause to strip theenzyme from the reaction surfaces. Hence, substantially non-uniform,noncontrollable reactions would occur. As a result, only partiallyreacted products could be obtained which then thereafter had to befurther treated or mixed to be normalized as to content, quality, etc.,to obtain the sought-after product.

In order to remove the disadvantages of enzyme stripping and low flowrates, various other approaches have been used such as pulsatingreactors which supposedly attempt to overcome the disadvantages of theprior art apparatus limitations, yet rely on the batch processadvantages of quiescent coagulation followed by pulsation.

Other attempts have been made to fluidize a reaction using fluid bedprinciples. For example, the enzyme is deposited on the fluidizedheterogeneous phase with a substrate forming the liquid phase beingreacted upon by the fluidized particles. As it is known from fluid bedmechanics, considerable abrasion exists between the fluidized particleswhich results in the loss of enzyme. Thus fixed and fluid bed reactorshave been suggested with various attempts made to improve the masstransfer, the rates and/or interface restrictions in the prior artreactors or processes.

Needless to say, these complications have introduced numerous problemssuch that the continuous cheese-making process has been a longsought-after goal. Hence the present process has as an objective theproduction of cheese at substantially improved rates, with high quality,in a consistent and controlled manner at either low or hightemperatures, with a tolerance for a high percentage of particulates inthe substrate, excellent rennet to milk ratios, repeatable, controlledprecise exposure to rennet, with substantial elimination of operatorerror, and also if desired a cheese without rennet being present.

THE PRIOR ART

In evaluating the present invention, the applicant is aware of thefollowing art:

U.S. Pat. No. 3,796,175 granted Oct. 30, 1973 to Berdelle-Hilge;

U.S. Pat. No. 3,884,641 granted May 20, 1975 to Kraffczyk et al.;

U.S. Pat. No. 3,945,310 granted Mar. 23, 1976 to Stenne;

U.S. Pat. No. 4,016,293 granted Apr. 5, 1977 to Coughlin et al.;

U.S. Pat. No. 4.048,018 granted Sept. 13, 1977 to Coughlin et al.;

U.S. Pat. No. 4,102,746 granted Jul. 25, 1978 to Goldberg;

U.S. Pat. No. 4,169,014 granted Sept. 25, 1979 to Goldberg;

U.S. Pat. No. 4,292,409 granted Sept. 29, 1981 to Cremonesi;

U.S. Pat. No. 4,416,993 granted Nov. 22, 1983 to McKeown.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

In accordance with the present invention, it has now been found thatrate quality, flow, and mixing constraints imposed by an enzymedeposited on a flat surface or suspended particle may be substantiallyavoided by employing a microporous sheet upon and in which the enzyme isimmobilized. Thus rennet is not in the final product. At the same time,microporosity is maintained in the sheet while the advantages of aflow-by reactor are maintained and, more importantly, the highparticulate fluids may also be treated at great advantage. The reactordesign and the advantages thereof have been disclosed in a companionapplication Ser. No. 595,954 filed Apr. 2, 1984, now U.S. Pat. No.4,689,302.

However, the benefits of the present process accrue as a result of thehighly controllable flow characteristics, the precise milk to rennetratios, the enzyme orientation or disposition, the enzymeimmobilization, the quality control, the porosity considerations, andthe flow distribution which is obtained by the process steps herein.Other advantages are, for example, yields which are highly controllable;quality which is repeatable; the freedom to make changes includingsaving milk (as rennet is not present until contacted with theimmobilized enzyme); the starter culture may be easily metered in acontinuous manner; renneting can be done independent of coagulation;renneting at low temperature can be followed by coagulation elsewhere;off line coagulation may be done later; renneting and coagulation may bedone in a continuous tube; renneting could also be done at almostcoagulation temperature; the pipe reactor can be shut down andreactivated without microbial contamination (following properprecautions), etc.

These advantages are obtained without the prior art disadvantages suchas clogging or blinding of the reactor surfaces. Still further, thegreat benefit vis-a-vis for example the filter type reactors, has beenrealized by the ability to process particulate containing substrateswhich contrariwise to the prior art shortcomings, do not substantiallystrip, abrade, or remove the immobilized enzymes. Rather, the presentprocess, as practiced, improves the flow characteristics, the fluiddistribution, and at the same time does not allow the particulates togrow to such size as to cause macro-curd formation.

In accordance with the present invention, it has also been found thatthe fluid distribution aids immeasurably to the production of a highquality product by the immobilized rennet, as precise milk to rennetratios may be computed, including obtaining precise time, temperature,and space velocity conditions.

While the exact mechanisms responsible for the reaction are not known,it is believed that the fluid flow properties interact with themicroporous, reticulated three-dimensional matrix in which theimmobilized rennet is held. It is postulated, without being bound bythis theory, that the individual liquid "cell" residence time in thereactor is beneficially improved by the microporosity, yet all the flowreactor, but more precisely, flow-by reactor process characteristics areretained.

It is believed that the flow characteristics are especially beneficialbecause the partially reacted or even reacted and not yet fullycoagulated fluid does not clog the surfaces, but rather strips thesewithout stripping the rennet. As a further benefit, a flow reactorconfiguration, as further explained herein and which forms one of theprocess embodiments, has been improved by the rib structure being alsoof a microporous nature and carrying the immobilized enzyme rennet. Amesh material likewise may be of a microporous nature carrying theimmobilized enzyme rennet and disposed in a reaction zone. Use of themesh device forms another embodiment herein. These flow division ordistribution means further aid and yet not impede the reaction causingthus the enzyme initiated reaction to continue in an advantageous mannerwith controlled turbulence further benefiting the process.

Turning now to the specific method whereby the immobilized rennet isobtained, in previous U.S. Patents such as No. 4,102,746 and No.4,169,014, the method of immobilization has been disclosed. Thesepatents are incorporated by reference herein and need not be discussedin greater detail. However, the immobilization technique employed inthese patents is useful for the employment of the present enzymes sothat the immobilization in the three-dimensionally reticulatedmicroporous structure can be obtained in a similar manner with excellentresults.

Based on the above disclosure and based on the studies as conducted, itis apparent that it is not merely the enzyme-reactor surface causativeeffect that plays a role, but also that the three-dimensional structurecontributes to the overall reaction in the fluid flow-by, acting throughthe in-depth deposited enzyme. In other words, there is a benefit whichis far greater than attributable to the surface catalyzed reactions. Theresults go beyond the expected results which are merely obtained fromthe surface catalyzed reactions and are gained by employing in thereaction three-dimensionally immobilized rennet which somehow has anability to influence the reaction conditions in the flow-by reactor, yetwithout any substantial drawbacks of the surface deposited enzymes, e.g.stripping, abrading, clogging, etc.

Based on these considerations and based on the various alternativeswhich are possible to employ, the further benefits accrue as a result ofthe following advantages. One, there is an improved ability to exhaustthe substrate constituents which may be usefully converted by theenzyme. The typical discharge of useful components in whey, e.g. whenmaking cheddar cheese, in accordance with the prior art, is about 89.4%of whey, of which water is 83.1%, of which the residue, 6.3% on dryweight basis, are useful. In accordance with the present process, bybeing able to react the particulate-containing substrate from whichcoagulated curd material has been substantially removed, the resultshave been improved such that substantially entirely the componentsconvertible to cheese by the action of rennet may be converted. Of thestill useful material left in the whey, this may, of course, be treatedin a different manner to convert it to different products, includingcheeses made from whey, e.g. Ricotta cheese.

As another benefit, the rate of coagulation of casein can be carefullycontrolled by raising the temperature. This characteristic may be usedin conjunction with or made dependent upon the flow rate. Thus thecoagulation condition and the subsequent coagulation may beappropriately controlled without affecting the rennet, the quality orconsistency of the cheese, etc. The temperature effect upon rennet underthese conditions is considerably better monitored and the enzyme isconsiderably less susceptible to damage, because the conditions areessentially adiabatic due to the temperature maintenance being capableof very careful control. Also staged reactors with different flow and/ortemperature conditions may be used.

One of the principal factors which has been found to affect the productand process has been the spacing of the sheets. For example, the rangefrom 0.050 to 0.003 inches has been found to be useful with 0.040 to0.005 being the preferred range for the critical spacing distance. Atthe lower end of the spacing range, the pressure drop becomes too great;at the higher end the renneting reaction is less susceptible to precisecontrol.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings in which similar elements are given similar referencecharacters and wherein various aspects and embodiments of the inventionare illustrated:

FIG. 1 is a side elevation, in section, of a jellyroll spiralconfiguration reactor constructed in accordance with the concepts of theinvention and with the spacing layer omitted for the sake of clarity;

FIG. 1a is a fragmentary bottom plan view of a material from which thespiral reactor of FIG. 1 is formed and showing the ribs formed on onesurface thereof;

FIG. 2 is a front elevational view of a portion of the spiral reactor ofFIG. 1 and showing the manner in which the ribs on one surface of thematerial keep adjacent convolutions of the reactor separated;

FIG. 3 is a fragmentary top plan view of a net-like spacing sheet whichcan be used with the support medium to construct a reactor in accordancewith the concepts of the invention;

FIG. 3a is a fragmentary side elevation of the spacing sheet of FIG. 3positioned adjacent one surface of the support medium;

FIG. 4 is a simplified side elevational view of the completed reactorwith inlet and outlet pipes connected thereto;

FIG. 5 illustrates the feed rate versus clotting (coagulation) time atvarious feed (substrate) temperatures;

FIG. 6 illustrates long term clotting effects at a certain flow rate ina continuous operation; and

FIG. 7 illustrates coagulation time versus flow rate (or residence time)at elevated temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A type of microporous sheet 30 is shown and described in U.S. LettersPat. No. 4,102,746 issued July 25, 1978 in the name of Bruce S. Goldbergand assigned to the assignee of the instant invention. In addition,other microporous plastic and rubber sheets can be used, e.g. as shownin U.S. Pat. No. 4,226,926, also assigned to the assignee of the instantinvention.

Proteinaceous preparations, for example enzymes, can be immobilized uponthe silica fragments on the surface of the sheeting and in the pores ofthe sheeting employing the techniques described in U.S. Pat. No.4,102,746 and using with material such as described in U.S. Pat. No.4,226,926. Other suitable techniques are also known in the prior art.

Because of the pore 32 size being in the range of 0.01 microns to 100microns, the presence of fat globules, protein, cheese solids and anymaterials which can coagulate to form globules can quickly seal thepores, and reduce or completely cut off any flow through the reactor.Since fat globules are larger than the pore size, either alone or with anumber combined, these globules soon coat the surface penetration andprevent any of the feedstock from entering into the pores. These fatglobules must be removed by filtration of the feedstock before it isintroduced into the reactor system or periodically the feedstock streammust be stopped and the reactor system backwashed by the use of suitablematerial, such as water, and causing it to flow reversibly through thereactor system, that is from surface to surface.

Turning now to FIG. 1, there is shown a spiral reactor 50 constructed toachieve the results of the present process. The reactor 50 comprises aporous core 52 of appropriate diameter and volume such that it canaccept the feedstock introduced into the reactor 50 or to receive theconverted feedstock to conduct it from the reactor 50. The spiralreactor 50 is substantially made up of microporous sheets 54 having aproteinaceous preparation immobilized on or in the pores of the sheets54. However, the sheets 54 are modified to include a plurality oflongitudinal ribs 56 on one surface thereon as is shown in FIGS. 1a and2. The ribs 56 contact the adjacent convolutions of the spiral to keepthem separated and to establish a series of passages 58 therebetween inconjunction with the spiral convolutions themselves. The height h (seeFIG. 2) is so chosen that a desired flow-through reactor 50 is attainedwhile insuring proper conversion of the feedstock. The height h of theribs 56 has been found to be in the range of 0.003" to 0.050" and mostparticularly about 0.005" to 0.040". The height will, of course, have tobe altered depending upon the feedstock employed. The total spaceavailable for feedstock flow is identified as the void volume andmeasured in cubic centimeters.

Alternatively, a spacing sheet 60 as shown in FIGS. 3 and 3a may beemployed instead of the raised ribs 56. The spacing sheet 60 is made ofa flexible plastic or rubber material and in an open fish net format toassure light weight and flexibility. The spacing sheet may be made of amicroporous material with an enzyme immobilized thereon. Spacing sheet60 is simply laid atop a microporous sheet 70 as shown in FIG. 3a. Eventhough the surfaces of the spacing sheet 60 are substantially flat, thesurface of the sheeting will rest on a number of peaks on the surface ofsheet 70, providing, along with the open net format of the sheet 60,sufficient passages to permit the feedstock to pass the spacing sheet60.

The reactor 50 is completed as shown in FIG. 4 by encapsulating theentire unit in a suitable housing 80. A pipe 82 is provided to gainaccess to the core 52 and plenum 84 is to provide access to the free endof the reactor. Although it is preferable to introduce the feedstockunder suitable pressure into the core 52 of the reactor 50 and removethe converted feedstock from the free end of the reactor 50, theopposite flow pattern can also be used, namely introduce the feedstockto the free end and remove the converted feedstock from the core 52.

The microporous sheeting together with a spacing layer, whether ribs 56or spacing sheet 60, are wound upon core 52 to form the spiralconfiguration shown in FIG. 1. As the feedstock flows along the passage58, it is sufficiently exposed to enzyme to convert the feedstock to ahigh degree.

The spiral reactor 50 may be formed of a stack of individual microporoussheets 54, which are about 0.020 inches thick. It may also be formed bywinding on itself a 90 inch length of 0.020 inch thick microporousmaterial to produce a layer 1 inch thick and having a width of 3 inches.Considering that both sides of the sheets 54 are available, a totalsurface area of 270 square inches, reduced by the areas of contact ofthe sheet 54 edges with the housing 80, about 240 square inches ofsurface is effective. Employing ribs 56 having a height h of 0.010inches and winding the sheets 54 on a porous core 52, 1 inch in diameterand 3 inches long, results in a reactor 50 having an outside diameter of2 inches and a void volume or flow path volume of between 30 and 40cubic centimeters. When potted, that is with the housing 80 in place,the outside diameter is 23/4 inches thick.

With the enzyme lactase immobilized on the sheets 54, as set out in U.S.Pat. No. 4,169,014 identified above, and employing skim milk adjusted toa pH of 5.1 at a flow rate of 10 milliliters per minute at 40° C. with aresidence time of 3 to 4 minutes, reactor 50 is capable of hydrolyzing90% of the lactose in the skim milk.

If a reactor 50 is constructed using a spacing sheet 60 of a fish netformat having open areas from 50-90% but preferred in the range of70-80% of the area of a continuous sheet of the same dimensions and athickness of 0.010 inches, the available void volume will be about 50cubic centimeters. Using the same feedstock under the same conditionsset out in the previous example, approximately the same percentage oflactose in the skim milk will be hydrolyzed. It has been found that inactual practice conversions of the feedstock have reached values as highas 90%. The flow path through the reactor 50 is assumed generally to belaminar, but in practice it has been found to have a random flow, theflow causes eddies, and in some cases may also pass through the sheetitself.

Various coagulation or clotting time studies at various temperatures andresidence times have been illustrated in the drawings. These will bedescribed in greater detail in the examples herein. In brief, the flowrate of the feed at the given temperature is indicated from which thecontact or residence time is calculated based on the reactor volume.Thereafter the actual clotting or coagulation time is correlated againat the particular temperature used. These data will be described andexplained below in the Examples as these relate to FIGS. 5, 6 and 7.

These and others benefits will be further pointed out herein below withreference to the specific embodiments wherein the examples illustratemerely the various aspects of the invention and are not intended tolimit the broader scope of the invention.

EXAMPLE 1

A calf-rennet milk clotting enzyme was immobilized on a spiral reactoras shown in FIG. 1 for testing on continuous coagulation of skim milk.The milk-clotting apparatus was set up for the coagulation time studyand the parameters of temperature and residence time were studied.

The calf-rennet clotting enzyme was immobilized on the spiral reactor byusing the standard immobilization technique described in U.S. Patentssuch as No. 4,102,746 and No. 4,169,014.

Using the milk-clotting enzyme immobilized spiral reactor, continuouscoagulation on skim milk was achieved. Skim milk was pumped through therennet immobilized reactor at various flow rates (residence times) andwas coagulated at 30° C. The obtained data were depicted in FIGS. 5 and6.

Residence time (flow rate), temperature and pH of skim milk were themajor factors that affect the coagulation time.

After six hours of continuous operation as shown in FIG. 6 at low feedtemperature (6° C.) and low flow rate (20 ml/min at 1.5 min. residenceor contact time) a slight loss of activity was observed, possibly due tothe deposition of protein on the surface. Higher feed-temperatures andhigher flow rates help to minimize substantially entirely this problem.Both microbial and calf rennet were tried. Protease enzymes such aspepsin or alkaline protease are also useful.

For this example, a commercially obtainable calf rennet clotting enzymewas used. It contains approximately 95% chymosin and 5% pepsin.

A spiral reactor, 3" high×1" ID×21/4" OD, as described previously, wasmade and immobilized with rennet enzyme using the standardimmobilization process mentioned above. Skim milk was used as thesubstrate.

A clotting test unit consists of a 30° C. water bath, a test vessel inwhich the treated substrate is held and which is rotated by an electricmotor. Rotation is at about 4-10 RPM.

The continuous coagulation system was run as follows. A vessel for milk,a cooling coil, and the above described reactor were enclosed in a waterbath which was then controlled at about 6°-8° C. and 14° C. and 19° C.,or at any other preselected temperature. The reactor was first flushedwith a pH 6.5 buffer (0.5 gm sodium acetate per liter), followed withskim milk at the desired temperature, with various flow rates todetermine the effect of both temperature and residence time on thecoagulation time being thereby observed. Skim milk was also pumpedthrough the reactor at a constant flow rate for a period of 6-8 hours tosee if any change on activity occurs.

The reactor was run each day for approximately six hours at one desiredtemperature. The reactor was then cleaned up with (a) 6.5 pH buffer (30min) and followed with (b) 6.5 pH buffer containing 0.05% hydrogenperoxide (30 min) and stored in refrigerator for the next day trial.

Clotting time tests were established as follows. Ten milliliters of thetreated skim milk effluent were collected from the reactor in the testvessel. The vessel was rotated at 4-10 RPM in 30° C. water to determineboth initial (flake on wall) and completed clotting time (gel-like onthe whole surface of the 10 ml). However, it was found that manyfactors, such as size of coagulum, effluent feed temperature, heattransfer rate, etc., affect the initial coagulation time results. Thecompleted coagulation time for 10 ml sample was used for this study.

FIG. 5 presented a series of clotting tests on the rennet reactor skimmilk samples which were run with various flow rates at differenttemperatures. Activity is expressed as clotting time (min.) which is thetime for a sample of skim milk (10 ml) to clot completely under 30° C.The results indicated that a continuous coagulation process is achieved.At 6°-8° C. milk feed temperature, it took 30 min. and 45 min. to clotthe treated milk with a flow rate of 20 ml/min. (1.5 min. residence,i.e. constant time) and 30 ml/min. (1. min residence time) respectively.If the feed temperature was increased to 19° C., the clotting time wasdecreased from 30 min to 20 min and 45 min. to 30 min. with same aboveflow rates. Thus, the clotting time can be controlled by regulating bothflow rate and feed temperature.

As shown in FIG. 6, skim milk (6°-8° C. temperature) was continuouslypumped through the reactor with a constant flow rate of 20 ml/min. (1.5min. residence time). The effluent was then collected at variousoperating hours for the clotting test. The results indicated that theclotting time increased from 20 min. at first hour to 30 min. after sixhours of continuous operation. This may indicate that some proteincontamination deposits on the surface of the reactor which causes aslight loss in activity.

EXAMPLE 2

Typically in the manufacture of cheese milk, either skim or whole milkis first pasteurized and then cooled to 32° C. (about 88° F.).Thereafter a starter culture and rennet are added. In the presentexample and as shown in FIG. 7, the clotting (i.e. coagulation time) hasbeen illustrated as a function of substrate temperature and clottingtemperature in the apparatus as described in Example 1. However, theadvantages of the high temperature operations, e.g. 30° C. and 37° C.,are evident as less cooling (or conversely heating) is necessary.

The procedure was as follows:

The calf rennet milk clotting enzyme was immobilized in the spiralreactor described above (size 3" long×21/4 O.D.) and run at 30° C. and37° C. reactor temperature using both store bought pasteurized skim andwhole milk as feedstocks. Coagulation tests were done at 30° C. and 40°C. using a 10 milliliter coagulation test procedure described inExample 1. From the data shown in FIG. 7, the following apply.

At 30° C. and 37° C., with a residence time of 0.75 min. and 1.5 min.,coagulation did not take place inside the rennet immobilized reactor. Itdid occur in the external clotting test tube at 30° C. The coagulationtook place inside the reactor at 30° C. if the skim milk was passedthrough the reactor system twice. This proves that by adjusting theresidence time (or reactor size), it is possible to control the desiredcoagulation time. However, part of the reaction may have been initiatedin the heating coil before the reactor, the material was enzyme treatedalready on the first pass.

Further, the clotting test at 40° C. gives a much faster clotting rate(approximately 30%) than at 30° C. The whole milk gave approximately 20%longer clotting time as compared to skim milk.

Based on operating conditions, milk can be processed through the reactorat 30° and 37° C. Coagulation or clotting external to or internal to thereactor was observed. The immobilized calf-rennet enzyme used in thereactor was approximately six months old.

FIG. 7 thus presents a series of test results at various reactor andfeed temperatures and the resulting clotting test results at differenttemperatures. As mentioned before, skim milk and whole milk were firstheated to 30° C. and 37° C. and passed through the reactor at the samerespective temperatures. No coagulation and pressure drop increase inthe reactor was observed. If the first pass treated milk is storedovernight in the refrigerator and then reheated and repassed through thereactor, the cogulation took place inside the housing and reactor, whichindicates that the residence time is still a major factor affecting theclotting rate.

A reactor system that fits into a commercial process for continuouscoagulation of 30° C. or 37° C. is practical. As shown in FIG. 7, theclotting test at 40° C. gave a much faster clot rate than at 30° C. Forexample, with a reactor residence time of 1 minute, it takes 26 minutesclot time at 40° C. as compared to 38 minutes at 30° C. Whole milk gaveslightly longer clot time as compared to skim milk.

Further, it was concluded that at 30° and 37° C. reactor temperatures,breakdown of casein occurs. Not to clot or coagulate in the reactor butexternal to it is practical. By adjusting the residence time, one maycause clotting or coagulation to occur in the immobilized enzymereactor.

As the reactor can tolerate particulates, it is evident that not onlycheese can be obtained by casein coagulation, but protein coagulation inthe whey may also be practiced. Appropriate whey protein enzymes areavailable for immobilization from commercial sources.

Still further, immobilization of rennet allows its use even afterprolonged storage (under appropriate conditions). The treated substrateis rennet free and rennet thereafter does not continue to cause furtherreactions in the product. Precise control, e.g. residence time, flowrate, temperature, are possible. As milk is free from rennet up to thetime it flows through the reactor, the unreacted milk flow may bestopped and diverted, thus allowing swing capacity for different cheesesand also for storage of unreacted milk. Moreover, starter culture may beadded shortly before the milk is introduced in the reactors. Hence notonly rennet under or over addition is avoided, but also starter contactand addition is easily and precisely controlled. Various rennetingcontact times at various temperatures at various coagulationtemperatures introduce also great flexibility in cheese makingprocesses.

What is claimed is:
 1. A continuous process for making cheesecomprising:continuously flowing a substrate containing coagulatableconstituents through a spiral flow path defined by a spirally woundsheet, said sheet containing within it an immobilized enzyme that causesthe substrate to coagulate; coagulating the substrate and converting theresultant coagulate into cheese; said flow path being between adjacentsurfaces of said spirally wound sheet containing said immobilizedenzyme, said sheet being microporous and having micro-pores thatinterconnect adjacent flow paths formed by said spirally wound sheet,said adjacent surfaces being spaced apart by a spacer that is spirallywound with said microporous sheet, and said spacer being selected from agroup consisting of: (a) a plurality of microporous ribs on saidspirally wound sheet disposed longitudinally to said flow path, and (b)a further sheet having an open net form.
 2. The process as described inclaim 1 wherein said spacer consists of said microporous ribs and alsocontains said immobilized enzyme.
 3. The process as defined in claim 1wherein said substrate is reacted in said flow path at a temperature atleast 5° C.
 4. The process as defined in claim 1 wherein the substrateis reacted in a first flow path as defined in claim 1 at onetemperature, and thereafter reacted again in a second flow path asdefined in claim 1 at a different temperature.
 5. The process as definedin claim 1 wherein precise, predetermined temperature and contact timebetween said substrate and enzyme are maintained.
 6. The process asdefined in claim 1 wherein precise ratios of substrate to enzyme asbased on contact time are maintained, thereby maintaining precisequality control.
 7. The process as defined in claim 1 wherein saidenzyme is rennet.
 8. The process as defined in claim 1 wherein saidenzyme is rennet and said substrate is milk.
 9. The process as definedin claim 1 wherein said rennet is calf rennet.
 10. The process asdefined in claim 1 wherein the substrate is milk and it is treated at atemperature of up to 40° F.
 11. The process as defined in claim 1wherein the pH of the substrate to be treated is at a value from 5 to6.7.
 12. The process as defined in claim 1 wherein partial reactiontakes place in said flow path and completion of said reaction andcoagulation is in a reactor therefor.
 13. The process as defined inclaim 1 wherein said spacer is said sheet having open net form.
 14. Theprocess as defined in claim 1 wherein the spirally wound microporoussheet is made from a polyethylene silica composition.
 15. The process asdefined in claim 1 wherein the spirally wound microporous sheet is madefrom a polyvinyl chloride-silica composition and said immobilized enzymeis immobilized on said silica.
 16. The process as defined in claim 1wherein the spirally wound microporous sheet is formed from asulfur-free cured natural rubber.
 17. The process as defined in claim 1wherein an enzyme reaction takes place at a residence time from about0.5 to 5 minutes.
 18. The process as defined in claim 1 wherein anenzyme reaction in said substrate is for a period of 0.5 to 10 minutes.19. The process as defined in claim 1 wherein the spacing betweenadjacent surfaces is between 0.003 and 0.050 inches.
 20. The process asdefined in claim 1 wherein prior to introduction of said substrate insaid flow path, said substrate is inoculated with a bacterial culture toachieve a desired acidity.